A photographer will arrange 6 people of 6 different heights for photograph by placing them in two rows of three so that each person in the first row is standing in front of someone in the second row.  The heights of the people within each row must increase from left to right, and each person in the second row must be taller than the person standing in front of him or her.  How many such arrangements of the 6 people are possible?

A.     5

B.     6

C.     9

D.     24

E.      36

这个题目已经出现过好几次了，之前也看过有关求解，但还是不会做，请高人指点啊～多谢

用穷举法、I。125 346 II。124 356 III。123 456 IV。134 256 VI。135 246

前三个数代表第一排，后三个数代表第二排。